Non-invasive monitoring of a patient's pulse is common in medical practice. One type of pulse monitor (plethysmograph) typically incorporates an incandescent lamp or light-emitting-diode (LED) to transilluminate, that is shine through, an area containing large amounts of blood. The light source is mounted to well-perfused flesh, such as a fingertip. Light is emitted and transilluminates the tissue. The amount of light passing through that tissue is measured using a photosensor. Changes between the light emitted by the light source and the light received by the photo-sensor are caused by changes in the optical absorption of the light by the blood perfusing the transilluminated tissue. Either broad-spectrum visual light or narrow bandwidth light in the red or infrared wavelengths can be used. The absorption of certain wavelengths is related to the oxygen saturation level of hemoglobin in the blood perfusing the transilluminated tissue. The variations in light absorption caused by change in oxygen saturations make possible direct measurement of the arterial oxygen content.
Instruments based on this principle have been designed that use two or more wavelengths to measure oxygen saturation and in some cases pulse rate.
A common problem with these types of oxygen sensors (oximeters) or pulse monitors (plethysmographs) is the incompatibility of their physical construction with the anatomy of the patient. A common plethysmograph monitor is a bulky rectangular sensor containing both a light-emitting-diode and a photo-sensor spaced approximately one quarter inch apart on the same side of the fleshy bottom portion of the fingertip (see FIG. 1A and 1B). This design suffers from a distortion of measurement commonly called motion artifact.
Motion artifact is due to differential motion between the sensor and the patient's finger as well as changes in pressure within the tissue. This type of design also suffers from poor signal pick-up during periods of low blood flow in the illuminated tissue. Low blood flow occurs when blood vessels constrict and/or when there is insufficient volume of circulating blood in the body. These conditions commonly occur during shock or periods of low body temperature. This particular type of construction has been used for measuring oxygen saturation with good technical results but the same problems with motion and loss of pulse. An additional problem with this design is that it is typically attached by a small hook and-loop strap type fastener of the type commonly sold under the trademark Velcro.RTM.. This attachment design is easily dislodged from the finger either accidentally or involuntarily, terminating measurement prematurely and often unexpectedly.
Alternatively, a clamp design has been used to measure a patient's pulse. This design consists of one or more light-emitting-diodes adjacent to one side of a fleshy appendage such as a finger. The light from the LEDs is received by a photo-sensor placed on the opposing fleshy side of the appendage (see FIG. 1C and 1D). This type of construction generally consists of a small spring-loaded clip which attaches like a common clothespin to the tip of a finger or similar appendage. This type of sensor attachment has been used in an oximeter as well as a plethysmograph. The advantage of the clamp type of sensor attachment is that the optical path traverses through the nail and entire fingertip. This technique optically penetrates the tissue of the patient more deeply than does the simple single-sided surface sensor discussed previously.
This clamp type of sensor attachment suffers from some of the same defects as the single-sided type of sensor attachment in that it often yields inaccurate measurement due to distortion caused by motion artifact and also tends to be inadvertently removed. Further, the clamp type sensor attachment has one additional and serious drawback: The spring-loaded pressure on the fleshy tissue over a period of time will cause reduction of blood flow to the tissue. Reduction of blood flow causes loss of pulse amplitude and thus loss of the optical signal to be measured. To minimize this constrictive effect of the clamp type attachment, the sensor must be adjusted or repositioned frequently, generally once or twice per hour. This drawback makes this sensor's construction unacceptable for long term, uninterrupted measurement.
The phenomenon of motion artifact has been mentioned. Plethysmographs and oximeters operate on the principle that light extinction between the light source and the photo-sensor is the sum of two effects. The first effect is non-variant light extinction by stationary tissue. This would include skin, skin pigment, bone, nail, hair and other non-moving components of the tissue bed being illuminated. Referring to FIG. 1E, one identifies the non-variant component 10 of light extinction from stationary tissue shown with fixed amplitude over time.
The second effect of light pulsatile extinction is the time-variant absorption due to pulsatile arterial blood supplying the illuminated tissue bed. Referring to 11 on FIG. 1E, one sees that this is a quasi-sinusoidal pulse wave riding on top of the constant component 10 of light extinction. It is this second component that affords direct and accurate measurement of oxygen saturation in pulsatile arterial blood flow.
A sensor with appreciable mass or high aspect ratio is prone to developing relative motion between the light source, the photo-sensor and the tissue from minor mechanical disturbance. This relative motion creates concomitant variations in the light transmission from source to sensor and thus grossly distorts the measurement of light extinction. When this motion occurs, variances of light transmission are erroneous indicators of light extinction. These extinction errors ultimately cause corresponding errors in oxygen saturation measurement, all as a result of discontinuous contact and other causes of relative motion between the light source, the photo-sensor, and tissue. A possible profile of such a variant motion is shown in FIG. 1E as component 12.
In FIG. 1E, the sensor has moved transiently from the exact place where it had been fastened. The sensor moves due to a combination of high inertia caused by its substantial mass and poor conformance with the supporting tissue. Movement of the finger by the patient or some external disturbance causes relative motion between the sensor and finger. The change in light transmission created by this motion appears as a change in "light extinction" with time, designated as component 12 in FIG. 1E. The measuring instrument designed to monitor light extinction cannot distinguish optical data introduced to the sensor by the relative motion of the sensor from the optical data introduced by blood pulsation that the instrument is designed to analyze. Confusion of the instrument's logic inevitably results in inaccurate analysis of data from the oximeter and consequently erroneous measurement of oxygen saturation.
It should be evident that in situations where the sensor has significant mass relative to the finger and does not conform to the skin, motion artifact occurs with virtually every motion of the patient. When it is remembered that the patient may be unconscious and/or undergoing body motion, this motion, producing the artifactual component 12 in FIG. 1E, creates a serious impediment to consistent accurate measurement.
During severe physiologic stress, such as hypotension (low blood pressure), hypothermia (low body temperature), and shock (low blood flow), the bodily response is to constrict blood vessels (vasoconstriction) in order to divert blood away from the extremities and away from the periphery (the skin surface) to maximize blood flow to central vital organs (e.g., brain, heart, liver, etc.).
The internal carotid arteries are the major vessels carrying blood to the brain.
The nasal septum is the location of terminal branches of the internal carotid artery, namely, the anterior and posterior ethmoidal arteries. The nasal septum is recognized as an excellent place to monitor blood flow to the brain both because of the copious blood supply in this area (to warm incoming air) and because the branches of the carotid artery (including the anterior and posterior ethmoidal arteries) are among the last locations in the human body to suffer vasoconstriction under stress conditions.
Physicians have used surface mounted optical pulse sensors (plethysmographs) and optical oxygen saturation sensors (oximeters) fastened to body appendages (fingers, toes, ear lobes) with great success in healthy patients but with less success in critically ill and compromised patients. These surface sensors use two basic configurations. The first configuration (FIG. 1) comprises a small box-shaped sensor mounted onto a patient's digit by a hook and-loop fastener, (e.g., the product sold under the trademark Velcro.RTM.. This design may suffer from unreliable measurement due to vasoconstriction and motion artifact. Motion artifact, which causes errant measurements, results from motion differential between the sensor and the flesh being interrogated; motion artifact can be induced by both voluntary and involuntary motion. Motion artifact causes relatively greater measurement errors when the desired pulse signal is very small during vasoconstriction.
Vasoconstriction is a narrowing of blood vessels resulting in a diminishing volume of blood flow to the tissue supplied by those vessels. Vasoconstriction commonly occurs when a patient suffers physiological shock resulting from trauma, accident, infection, or surgical complication. It also occurs when a patient, already in an intensive care unit, suffers further complications or worsening condition. Reduced pulse volume may also occur when an anesthesiologist deliberately induces very low blood pressure to minimize bleeding for a specific surgical operation. During vasoconstriction, there is less blood for the surface type sensor to measure. The result is a diminishing optical pulse signal and a relatively greater influence of motion artifact errors.
The second surface-type sensor configuration (FIG. 1D) that has been used to measure pulse and oxygen saturation consists of a spring-loaded clip shaped much like a clothes pin. This sensor is provided with a light source on one side of the clip and a photo detector on the other side to measure the degree of light extinction during transillumination by the blood flow in the tissue between the two sides of the clip. This second configuration is usually more effective than the first because the optical path, through the nail and the entire finger tip, penetrates much more deeply than the surface sensor (FIG. 1); however, vasoconstriction in critically ill patients coupled with the occluding spring pressure of the clip often results in insufficient pulse amplitude to reliably measure pulse or blood flow. Hence even a deep penetration surface sensor may not be useful in a critically ill or compromised patient.